Process of fabricating a portion of an optical fiber capable of reflecting predetermined wavelength bands of light

ABSTRACT

The invention covers a process of forming gratings in optical fiber. The gratings are formed at depth wherein the evanescent waves of the optical fiber are encountered. The process permits the fabrication of gratings in the optical fiber that reflect light of predetermined wavelengths. The grated optical fibers can be used as reflectors or interferometers when the gratings are used in pairs.

This is a continuation of application Ser. No. 546,608, filed Oct. 28,1983, now abandoned.

CROSS-REFERENCE TO RELATED APPLICATIONS

"Ruggedized Grated Optical Fiber", by J. E. Goodman et al, U.S. Ser. No.546,609, filed Oct. 28, 1983; "Process of Tuning a Grated Optical Fiberand the Tuned Optical Fiber", by D. C. Schmadel, Jr. et al, U.S. Ser.No. 546,610, filed Oct. 28, 1983; "Process and Apparatus for Measuringan Evanescent Field in an Optical Fiber", by D. C. Schmadel, Jr., U.S.Ser. No. 546,611, filed Oct. 28, 1983; "Optical Fiber Coating Apparatus,by J. E. Goodman, U.S. Ser. No. 546,617, filed Oct. 23, 1983; "EtchingFountain", by J. E. Goodman, U.S. Ser. No. 546,618, filed Oct. 28, 1983,now U.S. Pat. No. 4,469,544; and "Optical Fiber Holder", by J. E.Goodman, U.S. Ser. No. 546,619, filed Oct. 28, 1983.

This invention relates to optical fibers. More specifically, thisinvention relates to a process of fabricating a portion of an opticalfiber to reflect predetermined wavelength bands of light.

BACKGROUND OF THE INVENTION

In a gaseous medium such as air, gratings and mirrors are used tocontrol the direction and intensity of light. Light can also passthrough a solid medium such as an optical fiber. It would be highlydesirable to have a process of producing the effects of mirrors andgratings in the optical fiber. One of the methods of producing thegrating in the fiber was described by B. S. Kawasaki et al in OpticsLetters, Vol. 3, No. 2, pages 66-68 (August 1978). However, the Kawasakiet al paper was only applicable to gratings in fibers which have aphotosensitive core material and with a reflectivity, r, of about 0.6,and having a long interaction length of about 50 centimeters."Reflectivity" is defined as r² /i², where r is the peak amplitude ofthe electric field for light which was reflected within the core and iis the peak amplitude of the electric field which is within the core andincident on the grating. "Interaction length" is defined as that lengthmeasured along the fiber axis over which both the grating and theincident light extend. A long interaction length causes the reflectanceband to be very narrow spectrally. This limits the useful applicationsof the grating. Therefore, it would be highly desirable to have aprocess of forming gratings in an optical fiber which can exhibit longor short interaction lengths with either high, i.e, r>90%, or lowreflectivities.

SUMMARY OF THE INVENTION

This invention covers a process of forming gratings in optical fibers.The process permits the fabrication of grating dimensions on a fiberwhich exhibit a predetermined interaction length. This enables theinteraction length to be customized for a particular application such asoptical fiber hydrophones. The process involves removing the outerjacket of an optical fiber, etching the fiber to a depth whereinevanescent waves are encountered, coating the fiber with a photoresist,exposing the photoresist to a wavelength of light at which it reacts,developing the photoresist, removing the soluble portion of thephotoresist, etching the fiber with an ion beam, and removing theremaining photoresist. This permits the formation of gratings of manydesirable shapes and interaction lengths.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a holder for the optical fiber cable duringprocessing.

FIG. 2 illustrates a cut-away view of an etching fountain.

FIGS. 3 and 4 illustrate an optical fiber coating apparatus.

FIGS. 5 and 6 illustrate an apparatus for testing an evanescent field inan optical fiber.

DETAILED DESCRIPTION OF THE INVENTION

An optical fiber contains a central core through which the light travelssurrounded by a cladding of a material having a lower index ofreflection than the core. Usually, a protective jacket surrounds thecladding material. A suitable example of an optical fiber is a singlemode light polarization maintaining fiber of the Andrew Corporation. Togain access to the cladding and core, the protective jacket is removedwith a suitable stripping material such as Photoresist Remover 1112A, aproduct of the Shipley Corporation. The fiber is contacted with thephotoresist remover for a sufficient time to remove the protectivejacket. Elevated temperatures of about 50° C.-70° C. speed the reaction.At 60° C., the protective jacket is removed in about 5 to 10 minutes.

Removing the protective jacket exposes an optical fiber coated with alayer of indium. The indium material is supposed to hermetically sealthe fiber.

The exposed indium-coated fiber is fixed to a suitable holder for theadditional processing steps of my invention. Preferably, the holder isfabricated from a corrosion-resistant temperature suitable material suchas a ferronickel like Invar® [36% nickel and 64% steel (a carbon contentof 0.2%)], or other suitable metals or materials. A method of fixing theindium exposed portion of the fiber is with solder, such as indiumsolder. Of course, if the fiber does not have an indium coating, then anappropriate material capable of attaching the holder to the fiber shouldbe used. Any holder is suitable provided that the tension on the fibercan be adjusted and it also allows complete access to the exposedportion of the fiber. Initially, the tension is adjusted to maintain apositive tension on the optical fiber.

A preferred holder and optical fiber unit is illustrated as 10 inFIG. 1. Although used in this process with optical fibers, the holder isalso suitable for use in processes which involve fine wires, rods, andthe like, having diameters on the order of from about 5 to 100micrometers. The holdr 12 is fabricated from Invar® or other suitabletemperature-stable materials. Preferably, the holder 12 is a unitarypiece with the exception of adjusting means 14. The optical fiber 16 isfixed to the holder 12 with indium solder or by other suitable means.The holder 12 allows complete access to the exposed indium-coatedportion to the optical fiber 18 for subsequent processing. The holder 12has sections 12a and 12b which are thinner than the thickness of theholder 12. This design provides a means for the movement of the holder12 to tension the fiber 18 by the action of adjusting means 14 on thelever arm portion 12c of the holder 12 without twisting, i.e., movingout of a fixed plane. For example, if the holder is about 4.7 mm thick,then 12a and 12b should be about 1.1 mm across. In other words, 12a,12b, 12c and 14 act as a tensioning means for the fiber 18 whilemaintaining it in a fixed plane during tensioning. A suitable adjustingmeans 14 is a screw of Invar® or stainless steel. An adjustment of about0.5 mm is suitable for tensioning the fixed optical fiber section 18.Optionally, the holder 12 incorporates a means for attaching 12d, theholder 12 to subsequent processing apparatus as disclosed, for example,in FIGS. 3 and 4. More specifically, the notch 12d orients the holder 12and permits it to be attached to further processing apparatus in areproducible and accurate fashion. Suitable dimensions for the holder 12are about 50 mm from edge to edge across the top of the holder where thefiber 16 is connected (i.e., width) and about 43 mm for the lengthperpendicular to the width.

After the fiber is mounted in the holder, it is then placed intosuitable etching apparatus which contains a suitable indium etchantsolution such as 1 molar ferric chloride to remove the indium. Asuitable etching period is from about 3 to 10 minutes at roomtemperature. Alternatively, any commercially available copper printedcircuit board etchants, available from the Shipley Corporation, are alsosuitable. Any etching apparatus is suitable provided that it exposesonly the fiber and not the holder to the etching material.

A preferred etching apparatus is the etching fountain 20 illustrated inFIG. 2. The fountain 20 is preferred because it can be used with harshand dangerous chemicals, such as ferric chloride without a hood to ventthe fumes. The fountain 20 has an outer jacket 22 connected to a meansfor creating a downward flow of air such as a pump 23. The pump 23creates a downward air flow and removes the noxious vapors and gasesfrom the air space over the work piece. Inside the outer jacket 22 is acentral inner chamber 24 connected to a means for injecting a fluid intothe chamber 24 such as a tube 25. The tube 25 is connected to a meansfor circulating a fluid such as pump 29. Surrounding the inner chamber24 is a an outer overflow receptable 26 which is also connected to ameans for removing fluid from the overflow receptacle 26 such as a tube27. The tube 27 is also connected to the pump 29. The pump 29 circulatesan etchant fluid or solution 28. The upper part of the outer jacket 22has a means for holding a work piece such as a slit. The slit fits theoptical fiber holder 12 and situates the optical fiber 18 or other workpiece such as a fine wire or rod just above the top of the inner chamber24. The inner chamber 24 is designed so that the pumping action of pump29 exposes the indium coated portion of the fiber 18 to the solution 28but not the outer jacketed portion of the fiber 16. This prevents thesolution from being contaminated by the outer jacket 16. For the indiumetching, the pump 29 should deliver a uniform flow so that the indiumlayer is evenly removed. However, in the next step, when the innercladding is removed with a suitable cladding etchant to a depth whereinevanescent waves are encountered, the pump 29 should create anoscillating head of etchant solution so that the cladding ends taper tothe exposed center without sharp light leaking transitions. The taper isillustrated in FIG. 6. Of course, the etchant solution should notoscillate so violently that it is contaminated by the indium coating,i.e., this second etchant solution should only touch the cladding wherethe previous etching has removed the indium coating. A peristaltic pump29, as illustrated, is preferred for this application. Of course, 25must be flexible when 29 is a peristaltic pump. A suitable material isTygon® tubing. Since the etchants only touch the fiber 18, the holder 12may be but does not have to be constructed out of materials which areinert to the etchant solutions. The pump 23 keeps any noxious fumes awayfrom the holder 12 and people working in the area. Although the etchingfountain 20 is illustrated for one fiber, it can be scaled up toaccommodate any number of fibers, wires or rods. With one fiber, 22, 24and 26 are preferably tubular in shape, while in a more-than-one fiberdesign, 22, 24 and 26 are preferably rectangular in shape.

When the indium has been removed, the holder and fiber are removed fromthe fountain and rinsed in water and then placed into a second etchingfountain to etch away the outer cladding of the optical fiber to a depthwherein evanescent waves are encountered. A suitable etchant solution isfour parts of an ammonium bifluoride mixture with water and one parthydrofluoric acid with water, such as "BOE etchant (5-1)" available fromAllied Chemical Corporation in Morristown, N.J. A suitable etchingperiod is from about 3 to 8 hours. With the Andrew Corporation fiber,the core and cladding will have a diameter of about 66 micrometersbefore etching and about 6-10 micrometers after etching. The fountain isdesigned, as discussed above, to expose only that portion of the fiberfrom which the indium had been removed. The fountain is operated with apulsating or oscillating flow pattern to produce a fiber with a taper asillustrated in FIG. 6.

When the fiber has been sufficiently etched, which in the case of theAndrew Corporation fiber might be to a total diameter of about 6micrometers to about 12 micrometers and preferably about 8 to about 10micrometers, the fiber and holder are removed from the second etchingfountain, rinsed and then placed in a primer coating apparatus. Anexample of a suitable primer is C-55, HMDS primer, a product of theShipley Corporation. The priming takes from about 10 to 20 seconds. Anypriming apparatus is suitable provided that it primes only the etchedportions of the fiber. Preferably, the priming is done by vapordeposition. Alternatively, the coating apparatus described hereinaftercan be employed. The holder, the indium coating and other extraneousmaterials should not contaminate the primer material.

A preferred coating apparatus 30 is illustrated in FIG. 3 with a moredetailed section of the apparatus illustrated in FIG. 4. The apparatus30 has a control arm 32 which incorporates a fluid applicator head 34for photoresist primer or photoresist and the like. A suitableapplicator head 34 is a ruling pen nib. The surface tension of the fluid36 which is to coat the object such as primer and/or photoresist holdsit between the two nib halves which define the fluid reservoir until itis applied to the fiber 18. The control arm 32 is connected to areversibly rotating disc 42a mounted on a base 42. A control switch 44and power 46 provides the means for reversibly rotating the control arm32 in a reciprocating fashion. A guide wheel 40, such as a wheel orbearing, is attached to the lower portion of the control arm 32 as ittraces a pattern back and forth across a control track 38 having twoupper portions and a lower portion therebetween. Generally, the lowerportion of the control track 38 is configured so that about 5 to 25millimeters of the fiber are coated. Of course, it can be configured tocoat any length. When the wheel 40 is in the lower portion of the track38, the nib 34 containing the material 36 to be coated on the fiber 18straddles the fiber 18 while material 36 is deposited on the fiber 18 ascoated 36a. The thickness of the coating 36a is a function of theviscosity of material 36 in the nib 34 and the speed at which thecontrol nib 34 travels across the optical fiber. The track 38 isconfigured so that the material 36a only coats the etched portion of thefiber 18. The fiber is held by holder 12 and positioning means 48 in thecoating apparatus 30. The holder 12 is situated such that the object tobe coated is within the nib halves as the control wheel is in the lowerportion of the control track 38. Preferably, the lower portion of thetrack is configured so that the nib halves never touch the fiber 18. Theattaching and centering means 12d in holder 12 ensures that the fibers16 and 18 are held in the same position. The positioning means 48 isdesigned to surround and support the outside of holder 12 but leave thesurface holding the fiber 16 open for coating by the nib 32. The controlswitch 44 can be a manual switch which the operator merely reversed oran automatic switch to reverse the direction of the arm 32 after itmakes a pass over the fiber 18 and rises to an upper portion of thecontrol track 38. The drive for disc 42a can be any standard gear orbelt drive. The drive mechanism, not illustrated, is incorporated intothe control switch unit 44. Alternatively and not illustrated thecontrol arm can have a drive motor and guide track, as opposed to therotating disc, that will permit it to run parallel with the fiber 18 andthe control track 38. This apparatus is preferred because it overcomesthe surface tension problems of applying a fluid such as photoresist toa thin object, such as a wire, rod, optical fiber, and the like, whereinthe diameter is on the order of about 5 to 50 micrometers and moreusually on the order of about 10 micrometers for an optical fiber.

After priming, the holder and fiber are removed from the primingapparatus and placed in a photoresist coating apparatus. The photoresistcoating apparatus applies a positive photoresist such as Microposit1400-27® or Microposit 1400-33®, products of the Shipley Corporation.Any photoresist coating apparatus is suitable provided that it appliesphotoresist only to that portion of the fiber which has been primed. Apreferred apparatus is illustrated in FIGS. 3 and 4 and was describedabove. The photoresist is not applied to those portions of the fiberstill coated with indium, nor is the photoresist ever in contact withthe holder. This eliminates the possibility of contamination of thephotoresist coating. The coating operation as well as all followingoperations may be implemented in an amber light environment to avoidunwanted exposure of the photoresist. The amber light source shouldpreferably allow less than about 3 millijoules/cm² of light between 350nanometers (nm) and 480 nm wavelength to fall on the photoresistsurface.

Thereafter, the fiber and holder are removed from the photoresistcoating apparatus and the adjustable holder is adjusted so that thefiber is loose. If during the baking procedure the dimensions of theholder or fiber should change slightly, the fiber will not be stressedto the point of breaking.

Optionally, the fiber and holder are then placed into an oven for a softbake. For example, with Microposit 1400-27® or Microposit 1400-33®, thesoft bake consists of heating the fiber and holder in an oven for about20 to about 40 minutes and preferably 30 minutes at about 90° C. Afterthe soft bake, the fiber and holder are placed in a container to avoidexposure to cooler, ambient air, removed from the oven, and allowed tocool slowly. Thereafter, the fiber and holder are removed from thecontainer and the adjustable holder is readjusted so that the fiber istaut.

The fiber and holder are then placed in an exposure apparatus in whichthey are exposed to interfering beams from a light source such as akrypton laser, Model 3000-K produced by Coherent, Inc. The particularlaser and wavelength of light is selected to match the exposuresensitivity of the photoresist. A wavelength of about 413 nm is idealfor Microposit 1400-27® or Microposit 1400-33®. The exposure of thephotoresist to interfering beams of light creates a pattern wherein thephotoresist is exposed periodically along the fiber for a spatialseparation of exposure peaks or lines of about 0.3 micrometers.Preferably, the exposure peaks each lie in a plane perpendicular to thefiber axis. The light from the laser is spatially filtered by means of apinhole and then expanded and collimated. The exposure time is fromabout 1 to about 5 seconds for a 2-inch diameter beam of about 400milliwatts total power. When varying the grating dimensions, adetermination is made as to the desired grating size and then aninterference angle and a light source is selected which can create sucha pattern. Thereafter, a suitable photoresist is selected which issensitive to light of that wavelength.

After exposure, the fiber and holder are removed from the exposingapparatus and the holder is readjusted so that the fiber is loose. Thus,if the developer should be at a slightly different temperature therebycooling portions of the holder, it will not subject the fiber toadditional stress which might cause it to break.

Thereafter, the fiber and holder are placed into a developing apparatus.The developing apparatus is a fountain which contains a suitabledeveloper for the photoresist such as Developer No. 351 CD 23, a productof the Shipley Corporation. The fiber is exposed to the developer forfrom about 15 to 60 seconds, and preferably about 30 seconds, at roomtemperature. The developing apparatus is designed to expose thedeveloper to only those portions of the fiber which are coated withphotoresist. Thus, the developer will not be contaminated by, forexample, the indium on the fiber or the holder. The fiber is then rinsedin deionized water or other suitable rinses. The rinsing can take placeeither in a large bath where the fiber and holder can be submerged orthe fiber and holder can be washed in a fountain similar to the fountainused for the developer solution.

After the fiber is washed, it can then undergo an optional flat exposureprocedure at the same light intensity as the original exposure and for aperiod of from about 1 to 10 seconds. The flat exposure procedureconsists of exposing the fiber and holder to white light or light whichis within the absorbance band or the exposure band of the photoresist toexpose those portions of the photoresist which remain on the fiber andhave not been exposed by the original laser beam exposure. The fiber andholder are placed in an ion beam etching apparatus.

The ion beam etching apparatus uses a source of reactive ions such asfluorine ions derived from tetrafluoromethane or other suitable sources,or other suitable ions to etch the fiber and the photoresist. Thefluorine ions will etch that portion of the fiber which is not coveredwith the lines of the photoresist. Thus, the ion beam will etch gratingsinto the fiber. The etching takes from about 10 to 30 minutes.Generally, the gratings will be etched from about one-third of the wayto about one-half of the way around the circumference of the opticalfiber.

A preferred option requires that the fiber can be quickly etched in theion beam apparatus prior to the chemical etch and the priming andphotoresist coating to create a fiber with an off-center core where thecladding is thinner on one side. The thinner cladding area exposes moreevanescent wave and orients the fiber for the grating formation of thisthinner cladding section. The ion beam etcher need only remove about 1to 2 micrometers from a side of the fiber to create the necessary offsetto orient the fiber.

After the etching, the fiber and holder are removed from the ion beametching apparatus and placed in a photoresist removal apparatus. Thephotoresist removal apparatus is again a fountain which allows thephotoresist remover, such as Remover 1112A, a product of the ShipleyCorporation, to remove the remaining photoresist on the etched fiber.The photoresist is exposed to the remover for from about 10 seconds toabout 10 minutes. Alternatively, a solution of nine parts H₂ SO₄ and onepart 50% hydrogen peroxide at approximately 90° C. can be used as aremover. The fountain is designed so that only the portion of the fiberwhich contains photoresist is exposed to the photoresist remover. Thefinished fiber has a substantially uniform grating along its surface.The gratings on the fiber will reflect specific wavelengths of lightpassing through the fiber.

The amount of light the fiber reflects can be determined by adjustingthe previous steps to create a fiber having a predetermined gratingconfiguration. The closer the gratings are to the core, the greater thereflectivity of the gratings. Alternatively, the strength of thereflectance can be adjusted by adjusting the ion beam's etching time.The reflectivity or the amount of light the fiber will reflect isrelated to the depth of the grooves of the grating. The greater thedepth, the greater the amount of the reflected light. Finally, the angleof interference of the exposing light and the exposing light wavelengthdetermine the spacing of the gratings.

For example, a 0.3 micrometer grating peak spacing requires each of twointerfering beams to have propagation vectors which have an angle of 96°between them, or lie nearly in the same plane which contains the axis ofthe fiber, and have a bisector of the angle between them which is or isnearly perpendicular to the fiber axis. Taking into account therefractive index of the fiber, a 0.3 micrometer grating spacing willreflect light having a wavelength of about 8300 Angstroms. The 0.3micrometer spacing is peak to peak or valley to valley. The width of apeak or valley is about 0.15 micrometer and the depth from a peak to avalley is about 0.15 micrometer. The second of the interfering beams canbe derived from the first beam through the use of a beam splitter.Alternatively, the interference pattern can be produced by the use of amirror capable of producing Lipman fringes. The interference pattern iscreated by having the original beam interfere with itself.

If one wants to have a grating which will reflect a lower wavelengthlight, then the angle between the propagation vectors is adjusted to bemore than 96°, for example 100° or 110°. A smaller angle causes theexposed pattern in the photoresist to have a lower spatial frequency. Inother words, the separation between the exposed photoresist lines willbe greater.

More specifically, varying the process parameters permits thefabrication of gratings with interaction lengths that vary from about100 wavelengths to about 1 centimeter. The reflectivity r can be variedfrom about 0.05 to about 0.9 or higher. This wide reflectivity enablesto fabrication of gratings that can be adjusted to reflect wavelengthsof light between about 7000 Angstroms to about 20,000 Angstroms orhigher. Changing the laser lights source can extend the range below 7000Angstroms or above 20,000 Angstroms.

Furthermore, this process also allows one not only to change thespectral location of the light which is reflected by the grating butalso change the shape of the wavelength band. For example, the fiber canbe ion beam etched through shutters to reduce the exposure at the endsof the grating. This causes the strength of the reflectivity to varyalong the gratings, i.e., the center of the grating portion wouldexhibit a strong reflectivity and the ends would exhibit weakreflectivity. One could use shutters which vary the amount of the ionbeam to etch the grating so as to make gratings which might have agaussian dependency. Such an adjustment in the process permits thefabrication of gratings which have suppressed side lobes.

Optionally, the etching process can be monitored by the followingprocess to determine the amount and effect of cladding etching so as tobetter control the reflectivity of a grating. This process occurs duringthe etching chemical procedure and prior to actually forming the gratingon the fiber. The monitoring process requires interrupting the etchingchemical process, removing any etchant solution from the fiber,injecting light of nearly the wavelength of the desired reflectance bandinto an end of the fiber, contacting the etched fiber with a wicksaturated with a liquid having an index of refraction higher than thatof the fiber core material, for example, phenol, methyl salicylate(i.e., oil of wintergreen) and the like, and measuring the decrease inlight output from the fiber end opposite the injection end as caused bythe placement of the wick. The accuracy of the process is partlydependent upon knowing which size and contacting length on the fibertouched by the wick. This wick loss decrease, in light output, relatesto the strength of the exposed evanescent waves. The greater the lightloss, the more the evanescent waves are exposed.

FIGS. 5 and 6 illustrate an apparatus 50 for measuring the evanescentfield in an etched optical fiber 18. The optical fiber 16 and the etchedportion 18 thereof are held in a holder 12. The core of the etchedoptical fiber 18 is indicated as 18a. A container 52 holds a solution 54having a higher index of refraction than the etched optical fiber 18 inthe holder 12. Suitable materials are phenol, methyl salicylate, and thelike. A stable wick 56, fabricated from a material such as a urethanefoam polishing cloth, is placed in the solution 54. Preferably, the wickshould be at least as thick as the fiber 18, have known width, and notbe subject to expansion. Since the etched length of the fiber is usuallyabout 1 to 4 cm, a suitable width is from about 0.02 mm to about 0.1 mmand preferably is about 0.05 mm. Preferably, the wick should wet thesame length of the fiber as the width of the wick because the accuracyis partly based upon knowing the length of the fiber exposed to the highindex of refraction solution. To test the entire length of the etchedfiber 18, either the wick 56 must be movable along the fiber 18 or thefiber 18 and holder 12 must be movable along the wick. This movement canbe accomplished by a motor or person physically moving the fiber or thewick.

With the wetted wick 56 touching the fiber 18, one end of the fibercontains a holder system 58 capable of permitting light from a laser 60to be injected into the fiber 16. Any standard holder and lense focusingsystem capable of putting the laser light within the angle of acceptanceof the fiber is suitable. The laser should be selected to emit lighthaving a wavelength of interest for reflection when the gratings areformed. For example, if the gratings are to be designed to reflect lightat about 8300 Angstroms, then the laser can be a GaAs laser that emitslight having a wavelength of about 8300 Angstroms. The light passingthrough the fiber 16 containing the cladding and core unit 18 ismonitored on an optical power meter 62. Any suitable optical power metercan be used or a photodiode capable of converting the optical lightsignal into an electrical signal can be used. The signal obtained fromthe optical power meter 62 is recorded on a recorder 64 such as a chartrecorder.

As illustrated in FIG. 6, given a constant evanescent field, the greaterthe depth of the etching, the larger will be the wick light loss coupledout of the fiber 18 through the wick 56 containing the high index ofrefraction material 54 and the lower will be the recording on the chartrecorder 64. By moving either the fiber 18 or the wick 56 along theetched portion of the fiber 16, the uniformity and depth of etching canbe determined. Since the gratings will be formed within the fiber 18,the field will be at least as strong as the filed exposed during thetest. This process gives a representative correlation between thestrength of the exposed field after grating formation.

By repeating this monitoring process several times during the etchingprocess, the process also provides a method of monitoring the etch rateof the fiber. By etching many fibers to have different wick losses andthen forming gratings and measuring the reflectivity, the relationshipbetween wick loss for a known length of fiber and reflectivity for afinished grating can be determined empirically for a particularmanufacturer's fiber. Using this relationship, the reflectivity of agrating or grating portion can be approximately established immediatelyafter the cladding etching and before the grating formation.

The invention has been described with respect to preferred embodiments.However, it should be understood that the invention is not intended tobe limited in any way by the preferred embodiments. Modifications whichwould be obvious to the ordinary skilled artisan are contemplated to bewithin the scope of the invention.

What is claimed is:
 1. A process of fabricating gratings in an opticalfiber comprising:removing the outer layers of an optical fiber to adepth wherein evanescent waves are encountered; coating the fiber with aphotoresist; exposing the photoresist to interfering beams of lighthaving a predetermined spatial separation so as to fabricate a pluralityof exposure peaks in the photoresist; developing the photoresist;removing the soluble portions of the photoresist; and etching theoptical fiber with an ion beam.
 2. The process according to claim 1wherein the photoresist is a positive photoresist.
 3. The processaccording to claim 2 wherein the interfering beams of light are from acoherent light source.
 4. The process according to claim 3 wherein thecoherent light source is a krypton laser.
 5. The process according toclaim 4 wherein the optical fiber is coated with a primer material priorto coating with a photoresist.
 6. The process according to claim 4wherein the spacing of the exposure peaks is about 0.3 micrometer andthe fiber is capable of reflecting light having a wavelength of about0.83 micrometer.
 7. The process according to claim 5 wherein the coatingof the optical fiber with a primer is done by vapor deposition.
 8. Theprocess according to claim 4 wherein the optical fiber is ion beametched to offset a core within the optical fiber from the center of acladding surrounding said core prior to chemically etching the fiber toa depth wherein evanescent waves are encountered.
 9. The processaccording to claim 8 wherein the extent of etching is monitored byperiodically removing the fiber from the etchant solution and contactinga portion of the etched fiber with a wick containing a higher index ofrefraction material than the fiber while injecting light into an end ofthe optical fiber and monitoring the light output from the opposite endof the optical fiber.
 10. The process according to claim 8 which furthercomprises exposing the fiber with a flat exposure after developing theexposed photoresist.
 11. The process according to claim 10 which furthercomprises a soft bake of the photoresist prior to exposure.
 12. Theprocess according to claim 11 wherein the exposure is from about 1 toabout 5 seconds.
 13. The process according to claim 12 which furthercomprises removing the remaining photoresist on the fiber after ion beametching.
 14. The process according to claim 13 wherein the extent ofetching is monitored by periodically removing the fiber from the etchantsolution and contacting a portion of the etched fiber with a wickcontaining a higher index of refraction material than the fiber whileinjecting light into an end of the optical fiber and monitoring thelight output from the opposite end of the optical fiber.
 15. The processaccording to claim 14 wherein the coating of the optical fiber with aprimer is done by vapor deposition and preceeds the coating withphotoresist.
 16. A process of fabricating a grating in an optical fibercomprising:removing the outer protective jacket of an optical; etching aportion of the optical fiber having its outer protective jacket removedin an ion beam etcher for a sufficient time to fabricate an opticalfiber wherein the central core is offset with respect to the claddingover that portion of the optical fiber that was etched; etchingchemically the cladding of the optical fiber to a depth whereinevanescent waves are encountered; coating the etched portion of theoptical fiber with a photoresist primer; coating the primed opticalfiber with photoresist; exposing the photoresist to interfering beams oflight having a predetermined spatial separation so as to fabricate aplurality of exposure peaks in the photoresist; developing thephotoresist; removing the soluble portions of the photoresist; etchingthe optical fiber with an ion beam so as to form gratings in the opticalfiber; and removing the remaining photoresist on the optical fiber. 17.The process according to claim 16 wherein the spatial separation of theinterfering beams of light is adjusted to fabricate the plurality ofexposure peaks having a separation such that twice separation of thepeaks times the index of refraction of the cladding determines thewavelength at which light will be reflected by the gratings.
 18. Theprocess according to claim 17 wherein the peaks are adjusted to reflectlight having a wavelength of about 1.3 micrometers.
 19. The processaccording to claim 18 wherein the gratings are formed in that portion ofthe optical fiber closest to the outer portion of the cladding.
 20. Theprocess according to claim 19 wherein the gratings are formed from aboutone-third to about one-half of the way around the circumference of theoptical fiber.